Because there is no corneal contact, IOL Master lens constants will always be higher than those optimized for applanation A-scans in which there is a variable amount of corneal compression.
The IOL Master measures from the corneal vertex to the retinal pigment epithelium, but the axial length displayed by the IOL Master is adjusted by an internal algorithm to mirror immersion A-scan results with the exquisitely accurate Grieshaber Biometric System. In essence, the axial length displayed by the IOL Master is the equivalent of an upright, non-contact, ultra high resolution immersion A-scan. For this reason, IOL Master lens constants will be very close to those optimized for immersion A-scan ultrasonography, but always higher than those optimized for applanation A-scans.
If you already have optimized immersion A-scan lens constants, you can begin with these as a starting point.
Dr. Wolfgang Haigis, at the University of Würzburg has recommended the following approach for calculating the initial IOL Master SRK/T A-constant:
AUltrasound= Optimized A-constant for ultrasonography
ALIOL Master= Average IOL Master axial length
ALUltrasound = Average ultrasound axial length
For example, if your previously optimized applanation A-scan ultrasound constant for a specific IOL was 118.00, and for a specific patient the applanation A-scan axial length was 23.46 mm, and the IOL Master axial length was measured at 23.74 mm, then the SRK/T A-constant for the IOL Master would be:
118.00 + (3 * (23.74 - 23.56)) = 118.54
If you are making the transition from previously optimized applanation
A-scan lens constants to the IOL Master, below is a general rule of
thumb that will give you a reasonable starting point with one of
several popular IOL power calculation formulas.
Formula
Add to Applanation Lens Constant
Holladay 1
0.16
Holladay 2
0.16
Hoffer Q
0.16
SRK/T
0.34
SRK II
0.36
It is recommended that every 10 to 20 patients, the results be reviewed
and the lens constant for the formula being used be adjusted up, or
down, by no more than 0.10. This will prevent wide oscillations within
a relatively small sample size.
Click here to download an Excel spreadsheet that can be used to
initially optimize lens constants for the Holladay 1, SRK/T and Hoffer
Q formulas for patients whose axial length has been measured with the
IOLMaster. Simply enter data from 30 patients and an average lens
constant for each of these formulas will be calculated. This can also
be used as a starting point for IOL constant optimization.
Please check back at regular intervals for updates. Zeiss IOL Master
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